Objects rendering well in Shading but not in Layout
Lookdev (first image in the question) uses an environment lightning. This is not the case for rendered (second image) are you are supposed to choose the environment lightning yourself. 

See the button with circles on them, on top right of the 3D view:



If you want to have the same lightning in 'rendered' viewport shading, you can use the same texture that can be found in your installation path: \Blender Foundation\Blender\2.80\datafiles\studiolights\world.

And set the world surface like this, where forest.exr is from the folder above:



Though, the texture will be visible in the background and this is not the case in look dev shading mode.

If you don't want it, there is two (at least?) possibilities:

Either set the film to "transparent":



and the background will be transparent...

Or enhance the world node setting, using another color of your choice when the light ray goes directly to the camera: